Automatic article stacking apparatus have been typically employed in the past to sort such articles as letter mail. One such apparatus is known by the name "Letter Sorting Machine" (LSM). In the letter sorting machine, the letters are individually transported in an upright or vertical attitude within separate compartments in a carrier. The carrier compartment configuration resembles that of a file drawer. It contains several vertical separators located on a uniform pitch of about two inches. Each compartment of the carrier has a hinged floor which is capable of independent operation as a trap door. One compartment will contain only one piece of mail. The individual carriers are pulled or caused to be moved serially in a continuous train in such a manner that the direction of motion is substantially normal to the principal plane of the compartments. The path of the carrier is such that it takes it over a drop point corresponding to each destination for certain letters. At a predetermined time, a control system causes a programmed release of the trap door of a compartment and the associated piece of mail.
After its release through the compartment trap door the mail piece accelerates in free fall while maintaining the forward velocity imparted to it by the carrier. This mail piece, which has both a forward and downward velocity component, then impacts with a short curved slide and immediately descends down the slide into a small receiving compartment with a sloped floor that is provided for accumulating and stacking successively arriving pieces of mail. The arriving piece of mail is initially arrested by contact with that wall of the receiving compartment which is opposite the end of the chute. After the mail piece contacts the wall of the compartment, it drops on top of whatever mail has previously accumulated in the compartment. Commonly, the floor of the compartment is located on a slope which is directly downward and away from the end of the chute and as a consequence, this causes the leading edge of successive pieces of mail to remain in contact with the down stream portion of the wall to thereby obtain a flush condition on one face of the stack.
In a typical case, the stack will accumulate up to a level which is slightly below the end of the chute and at this point the stack will block the beam of a photosensor. The photosensor will then cause a conventional alert signal to be generated to alert personnel in the area to remove the full stack of mail. Removal of the full stack of mail is usually easily accomplished by grasping the stack in one or both hands since the stack height is usually manageable and the mail usually consists of relatively uniform pieces which are less then eleven by six inches in size.
There are a number of deficiencies with this previously described prior art system that is in current use. In this prior art system there is an unnecessarily severe impact between the article and the slide, involving not only the inevitable vertical velocity component due to the gravity induced transfer but also a forward or horizontal velocity component imparted by the transport carrier motion. Furthermore, in this prior art apparatus after the articles have been stacked the edges of the articles are flush only along one side of the stack rather than two sides. Consequently, in order for letter mail thus accumulated to be subsequently separated one piece at a time from the top of the stack by automated means typically used in mail processing systems, the mail must be "edged", that is, manually manipulated or introduced into a vibrating environment so as to achieve a flush edge condition on two sides of the stack. This step represents additional expense in the processing operation.
Furthermore, since the edges of the articles in this prior art apparatus are only flush on one side of the stack, this places some definite limitation on the range of random size articles that can feasibly be stacked without very troublesome procedures that will be required for a subsequent edging operation. For instance, when the article dimensions are less than half the corresponding compartment dimensions a succession of such articles may initially fail to stack one upon the top of the other but rather will fall side by side. If this occurs, subsequent vibratory efforts to edge such articles in a stack exhibiting such a condition will have indifferent success without the reconstruction of the stack piece by piece. Furthermore, should any of these articles be loose-leaved such as magazines or newspapers or the like, vibratory edging efforts are not only likely to fail but in fact may lead to damage of the leaves of the articles.
In addition, since the subject prior art apparatus provides a resulting stack which has the edges of the articles flush on only one side, this places an inherent limitation on the randomness of the article size that can be accommodated in the same stack as well as a limitation on the height of the stack that can be handled, without the resulting stack instability leading to an unacceptable risk of deshevelment, collapse and article damage.
The article stacking apparatus of the present invention overcomes these disadvantages associated with prior art systems and provides an article stacking apparatus that has reduced article impact velocity and which results in a stack of articles which has two flush edges without any need for additional edging operations.